1. Field of the Invention
The present invention finds one particular application in connecting a flat bundle of optical fibers to an integrated optical device.
2. Description of the Prior Art
One method for making such connections that is known in itself uses a flat bundle of planar waveguides formed for this purpose in layers of doped silica deposited onto a silicon substrate. At the same time housings are hollowed into the substrate with surfaces that follow crystal planes of the substrate and therefore enable accurate location of the optical fibers to couple them to the respective planar waveguides. This method that is known in itself is described in more detail in the remainder of the present description. It has the disadvantage that it does not produce good optical coupling between planar waveguides and the optical fibers when it is required to deposit the layers of silica by a relatively low cost method of deposition that is known in itself. The present invention is a result of discovering the causes of this drawback and consequently provides a remedy to this drawback. Nevertheless, it seems that these causes can give rise to similar disadvantages in other circumstances and that the same remedy can then be used with advantage. This is why the invention is first described hereinafter in a more general manner and then in one particular application.
Firstly, the invention consists in a method of fabricating a component having a crystalline silicon substrate, the method including the steps of:
depositing a layer of silica onto a crystalline silicon substrate, this silica layer being doped with dopants,
eliminating said doped silica layer over a region to be treated of said substrate, and
treating said substrate in said region to be treated so that the quality of the fabricated component is conditioned by the quality of the crystal lattice of said substrate in said region. This treatment can be of various kinds, as explained hereinafter. Regardless of the nature of the treatment, before the deposition of said layer of doped silica, the method includes a step of forming on said substrate a barrier layer of a barrier material opposing diffusion of said dopants, said doped silica layer being deposited onto said barrier layer at a high temperature such as that of flame hydrolysis, said barrier layer being eliminated in said region to be treated before said treatment step.
The barrier material is preferably a silica containing none of said dopants, at least those of the dopants of the doped silica layer that could degrade the useful qualities of the substrate. This silica is typically pure silica. The barrier layer is then advantageously formed by oxidation of the substrate. This layer can be made 400 nm thick by exposing the substrate to an oxygen atmosphere at 1350xc2x0 C. for 1 h, for example. However, this layer could equally well be formed by the flame hydrolysis deposition (FHD) process or by the plasma enhanced chemical vapor deposition (PECVD) process. At least one other barrier material may be used: silicon nitride.
This invention finds typical applications in the fabrication of optical and electro-optical components, the doped silica being used to guide light waves, the crystalline silicon being used for its optical, electrical, thermal conduction or ease of etching properties. The invention is in particular a result of the fact that it has been found that the crystal lattice of the silicon can be gravely disrupted by the diffusion of dopants which, in methods of fabricating optical components that are known in themselves, are included in a layer of silica deposited onto a silicon substrate. It is also a result of the fact that the rates of diffusion of such dopants are much lower in the barrier materials proposed than in silicon, for example one hundred times lower in the case of silica. These rates increase with temperature and, for the usual dopants, they become high only above 1000xc2x0 C. The present invention therefore finds applications when the substrate carrying the layer of doped silica must or can be heated to a high temperature at a time when treatment that necessitates good crystalline qualities of the substrate has not yet been carried out.
The invention will be usefully employed in certain industrial processes in which a step of treatment of this kind of a crystalline silicon substrate is prevented or merely rendered ineffective, difficult or costly when the substrate has been covered with a layer of doped silica. These processes are those in which the temperature reached during deposition of the doped silica causes diffusion into the silicon of dopants from this layer and in which such diffusion in turn causes degradation of the qualities that it is intended to exploit. In this context, the important characteristics include:
the natures of said dopants and their required concentrations in the layer of doped silica, and
the foreseeable temperatures to which the substrate is exposed and the foreseeable time periods for which it is exposed to such temperatures, and
the nature and the methods of the treatment to be effected.
Insofar as the natures and the concentrations of the useful dopants are required, boron and phosphorus may be cited in concentrations in the order of one molar percent relative to the silica. The function of these dopants is to reduce the viscosity of the silica to enable its temperature of use to be reduced or to enable the refractive index of the silica to be modified, for example. This index is reduced by approximately 5xc3x9710xe2x88x924 for each molar percentage point in the case of boron or phosphorus.
Other dopants that can be used for other functions include germanium, titanium, fluorine, chlorine, nitrogen, etc.
Where the foreseeable temperatures are concerned, it may be mentioned that the present invention enables the layer of doped silica to be deposited by the FHD process. This process, which is known in itself, has the advantage of being relatively economical but the disadvantage of heating the substrate to a temperature of 1350xc2x0 C. for one hour. A significant temperature rise could also be required if the FHD process were replaced with the PECVD process, for example.
Where the treatment to be effected on the substrate is concerned, a typical treatment is guided etching, said region to be treated then being a region to be etched. Etching of this kind is effected by exposing the substrate to an etchant that is xe2x80x9cguidedxe2x80x9d in the sense that the silicon is preferentially etched by the etchant parallel to the crystal planes of the substrate, so that the etching is guided by these planes. The etching rate is then typically much higher parallel to these planes than perpendicular to them. This etching exposes one or more crystal planes to exploit the fact that these planes have precisely defined relative orientations.
Using this typical method, the barrier layer is formed on a plane surface oriented along a crystal plane of the substrate. After elimination of the layer of doped silica and of the barrier layer over at least the region to be etched of this plane face, the etching processing steps are as follows:
definition of a guided etchant adapted to etch the substrate in a manner guided by crystal planes of the substrate,
application to this region to be etched of a layer resistant to said guided etchant and having at least one definition edge oriented in a crystal direction of the substrate, and then
exposing said plane face to said guided etchant to expose at least one crystal plane of the substrate from said definition edge, this plane forming a non-zero dihedral angle with this face. There is typically formed in this way, between two crystal planes exposed in this way, a locating Vee enabling precise orientation of an optical fiber on a silicon substrate. Crystal planes exposed in this way could have other functions, however, for example they could constitute mirrors to reflect infrared light guided in a layer of silica formed on the substrate.
In the case of another kind of treatment to be effected on the substrate, electrically conductive tracks, for example gold tracks, are formed on the latter to energize an active component such as a laser attached to the substrate. The dielectric constant of the material of the substrate must then be homogeneous and predictable. It has been found that this constant is seriously and erratically modified by the presence of impurities disrupting the crystal lattice. Consideration may also be given to the use of the semiconductor properties of the crystalline silicon.
One embodiment of the present invention is described hereinafter with reference to the accompanying diagrammatic drawings. If an item appears in more than one figure, it is always designated by the same reference symbol. The photosensitive resins employed are not shown.